The present invention relates generally to integrated circuit designs, and more particularly to methods and systems for implementing multiple electrical fuses in a fuse cell equipped with only one programming device.
Electrical fuses are often utilized for modern semiconductors. Typically, they are designed to blow when a current through the fuses exceeds a pre-determined threshold. When the fuses are programmed or “blown”, although not necessarily physically broken, they enter into a high impedance state. Electrical fuses are commonly used for making adjustments and repairs that are performed as late as after the chip is packaged. Since wirings are allowed at the two ends of the fuses, the fuses can be flexibly positioned within the chip, which is much more desirable than the conventional laser fuses as it is impossible to implement many metal layers or thick dielectrics above the laser fuses. This flexibility makes electrical fuses a desirable component for higher density memory devices.
However, conventional methods of programming electrical fuses in a memory device are not very efficient in utilizing precious silicon areas and thus are costly. For example, conventional methods for programming an electrical fuse require that one programming device is assigned for each fuse. In order to program an electrical fuse, a large supply current is necessary to be directed through the fuse. In order to provide this large supply current, programming devices attached to the fuses are very large. As the number of electrical fuses increase, the number of these large programming devices also increases
FIG. 1A presents a schematic diagram 102 showing how an electrical fuse is programmed using a NMOS programming device, while FIG. 1B presents a schematic diagram 104 showing how an electrical fuse is programmed using a PMOS programming device. In the schematic diagram 102, an electrical fuse 106 is placed between a NMOS programming device 108 and a high voltage source VDDQ. In the schematic diagram 104, a PMOS programming device 110 is placed between an electrical fuse 112 and a high voltage source VDDQ. A “Select” control signal enters through either a select line 114 or a select line 116, when either the electrical fuse 106 or 112 is assigned to be programmed.
The programming devices 108 and 110 are large in physical size since large currents are required to program electrical fuses, such as the electrical fuses 106 and 112. The conventional method of programming electrical fuses requires one program device for each fuse. This method is extremely inefficient and costly since each fuse requires a separate programming device. In a large array of fuses, the large programming devices will take up massive areas within a silicon environment.
It is always desirable to provide an improved programming mechanism with multiple fuses to improve silicon area utilization without causing deterioration to operational performance.